Transaction Execution Approval Language (TEAL)

TEAL is a bytecode based stack language that executes inside Algorand transactions to check the parameters of the transaction and approve the transaction as if by a signature. Programs have read-only access to the transaction they are attached to, transactions in their atomic transaction group, and a few global values. Programs cannot modify or create transactions, only reject or approve them. Approval is signaled by finishing with the stack containing a single non-zero uint64 value.

TEAL programs should be short and run fast as they are run in-line along with signature checking, transaction balance rule checking, and other checks during block assembly and validation. Many useful programs are less than 100 instructions.

The Stack

The stack starts empty and contains values of either uint64 or bytes (bytes are implemented in Go as a []byte slice). Most operations act on the stack, popping arguments from it and pushing results to it.

The maximum stack depth is currently 1000.

Scratch Space

In addition to the stack there are 256 positions of scratch space, also uint64-bytes union values, accessed by the load and store ops moving data from or to scratch space, respectively.

Execution Environment

TEAL runs in Algorand nodes as part of testing a proposed transaction to see if it is valid and authorized to be committed into a block.

If an authorized program executes and finishes with a single non-zero uint64 value on the stack then that program has validated the transaction it is attached to.

The TEAL program has access to data from the transaction it is attached to (txn op), any transactions in a transaction group it is part of (gtxn op), and a few global values like consensus parameters (global op). Some "Args" may be attached to a transaction being validated by a TEAL program. Args are an array of byte strings. A common pattern would be to have the key to unlock some contract as an Arg. Args are recorded on the blockchain and publicly visible when the transaction is submitted to the network.

A program can either authorize some delegated action on a normal private key signed or multisig account or be wholly in charge of a contract account.

  • If the account has signed the program (an ed25519 signature on "Program" concatenated with the program bytes) then if the program returns true the transaction is authorized as if the account had signed it. This allows an account to hand out a signed program so that other users can carry out delegated actions which are approved by the program.
  • If the SHA512_256 hash of the program (prefixed by "Program") is equal to the transaction Sender address then this is a contract account wholly controlled by the program. No other signature is necessary or possible. The only way to execute a transaction against the contract account is for the program to approve it.

The TEAL bytecode plus the length of any Args must add up to less than 1000 bytes (consensus parameter LogicSigMaxSize). Each TEAL op has an associated cost estimate and the program cost estimate must total less than 20000 (consensus parameter LogicSigMaxCost). Most ops have an estimated cost of 1, but a few slow crypto ops are much higher.

Execution modes

Starting from version 2 TEAL evaluator can run programs in two modes: 1. Signature verification (stateless) 2. Application run (stateful)

Differences between modes include: 1. Max program length (consensus parameters LogicSigMaxSize, MaxApprovalProgramLen and MaxClearStateProgramLen) 2. Max program cost (consensus parameters LogicSigMaxCost, MaxAppProgramCost) 3. Opcodes availability. For example, all stateful operations are only available in stateful mode. Refer to opcodes document for details.

Constants

Constants are loaded into the environment into storage separate from the stack. They can then be pushed onto the stack by referring to the type and index. This makes for efficient re-use of byte constants used for account addresses, etc.

The assembler will hide most of this, allowing simple use of int 1234 and byte 0xcafed00d. These constants will automatically get assembled into int and byte pages of constants, de-duplicated, and operations to load them from constant storage space inserted.

Constants are loaded into the environment by two opcodes, intcblock and bytecblock. Both of these use proto-buf style variable length unsigned int, reproduced here. The intcblock opcode is followed by a varuint specifying the length of the array and then that number of varuint. The bytecblock opcode is followed by a varuint array length then that number of pairs of (varuint, bytes) length prefixed byte strings. This should efficiently load 32 and 64 byte constants which will be common as addresses, hashes, and signatures.

Constants are pushed onto the stack by intc, intc_[0123], bytec, and bytec_[0123]. The assembler will handle converting int N or byte N into the appropriate form of the instruction needed.

Named Integer Constants

OnComplete

An application transaction must indicate the action to be taken following the execution of its approvalProgram or clearStateProgram. The constants below describe the available actions.

Value Constant name Description
0 NoOp Only execute the ApprovalProgram associated with this application ID, with no additional effects.
1 OptIn Before executing the ApprovalProgram, allocate local state for this application into the sender's account data.
2 CloseOut After executing the ApprovalProgram, clear any local state for this application out of the sender's account data.
3 ClearState Don't execute the ApprovalProgram, and instead execute the ClearStateProgram (which may not reject this transaction). Additionally, clear any local state for this application out of the sender's account data as in CloseOutOC.
4 UpdateApplication After executing the ApprovalProgram, replace the ApprovalProgram and ClearStateProgram associated with this application ID with the programs specified in this transaction.
5 DeleteApplication After executing the ApprovalProgram, delete the application parameters from the account data of the application's creator.
TypeEnum constants
Value Constant name Description
0 unknown Unknown type. Invalid transaction
1 pay Payment
2 keyreg KeyRegistration
3 acfg AssetConfig
4 axfer AssetTransfer
5 afrz AssetFreeze
6 appl ApplicationCall

Operations

Most operations work with only one type of argument, uint64 or bytes, and panic if the wrong type value is on the stack. The instruction set was designed to execute calculator-like expressions. What might be a one line expression with various parenthesized clauses should be efficiently representable in TEAL.

Looping is not possible, by design, to ensure predictably fast execution. There is a branch instruction (bnz, branch if not zero) which allows forward branching only so that some code may be skipped.

Many programs need only a few dozen instructions. The instruction set has some optimization built in. intc, bytec, and arg take an immediate value byte, making a 2-byte op to load a value onto the stack, but they also have single byte versions for loading the most common constant values. Any program will benefit from having a few common values loaded with a smaller one byte opcode. Cryptographic hashes and ed25519verify are single byte opcodes with powerful libraries behind them. These operations still take more time than other ops (and this is reflected in the cost of each op and the cost limit of a program) but are efficient in compiled code space.

This summary is supplemented by more detail in the opcodes document.

Some operations 'panic' and immediately end execution of the program. A transaction checked by a program that panics is not valid. A contract account governed by a buggy program might not have a way to get assets back out of it. Code carefully.

Arithmetic, Logic, and Cryptographic Operations

For one-argument ops, X is the last element on the stack, which is typically replaced by a new value.

For two-argument ops, A is the previous element on the stack and B is the last element on the stack. These typically result in popping A and B from the stack and pushing the result.

ed25519verify is currently the only 3 argument opcode and is described in detail in the opcode refrence.

Op Description
sha256 SHA256 hash of value X, yields [32]byte
keccak256 Keccak256 hash of value X, yields [32]byte
sha512_256 SHA512_256 hash of value X, yields [32]byte
ed25519verify for (data A, signature B, pubkey C) verify the signature of ("ProgData" || program_hash || data) against the pubkey => {0 or 1}
+ A plus B. Panic on overflow.
- A minus B. Panic if B > A.
/ A divided by B. Panic if B == 0.
* A times B. Panic on overflow.
< A less than B => {0 or 1}
> A greater than B => {0 or 1}
<= A less than or equal to B => {0 or 1}
>= A greater than or equal to B => {0 or 1}
&& A is not zero and B is not zero => {0 or 1}
\|\| A is not zero or B is not zero => {0 or 1}
== A is equal to B => {0 or 1}
!= A is not equal to B => {0 or 1}
! X == 0 yields 1; else 0
len yields length of byte value X
itob converts uint64 X to big endian bytes
btoi converts bytes X as big endian to uint64
% A modulo B. Panic if B == 0.
\| A bitwise-or B
& A bitwise-and B
^ A bitwise-xor B
~ bitwise invert value X
mulw A times B out to 128-bit long result as low (top) and high uint64 values on the stack
addw A plus B out to 128-bit long result as sum (top) and carry-bit uint64 values on the stack
concat pop two byte strings A and B and join them, push the result
substring pop a byte string X. For immediate values in 0..255 M and N: extract a range of bytes from it starting at M up to but not including N, push the substring result. If N < M, or either is larger than the string length, the program fails
substring3 pop a byte string A and two integers B and C. Extract a range of bytes from A starting at B up to but not including C, push the substring result. If C < B, or either is larger than the string length, the program fails

Loading Values

Opcodes for getting data onto the stack.

Some of these have immediate data in the byte or bytes after the opcode.

Op Description
intcblock load block of uint64 constants
intc push value from uint64 constants to stack by index into constants
intc_0 push constant 0 from intcblock to stack
intc_1 push constant 1 from intcblock to stack
intc_2 push constant 2 from intcblock to stack
intc_3 push constant 3 from intcblock to stack
bytecblock load block of byte-array constants
bytec push bytes constant to stack by index into constants
bytec_0 push constant 0 from bytecblock to stack
bytec_1 push constant 1 from bytecblock to stack
bytec_2 push constant 2 from bytecblock to stack
bytec_3 push constant 3 from bytecblock to stack
arg push Args[N] value to stack by index
arg_0 push Args[0] to stack
arg_1 push Args[1] to stack
arg_2 push Args[2] to stack
arg_3 push Args[3] to stack
txn push field from current transaction to stack
gtxn push field to the stack from a transaction in the current transaction group
txna push value of an array field from current transaction to stack
gtxna push value of a field to the stack from a transaction in the current transaction group
global push value from globals to stack
load copy a value from scratch space to the stack
store pop a value from the stack and store to scratch space

Transaction Fields

Index Name Type Notes
0 Sender []byte 32 byte address
1 Fee uint64 micro-Algos
2 FirstValid uint64 round number
3 FirstValidTime uint64 Causes program to fail; reserved for future use
4 LastValid uint64 round number
5 Note []byte
6 Lease []byte
7 Receiver []byte 32 byte address
8 Amount uint64 micro-Algos
9 CloseRemainderTo []byte 32 byte address
10 VotePK []byte 32 byte address
11 SelectionPK []byte 32 byte address
12 VoteFirst uint64
13 VoteLast uint64
14 VoteKeyDilution uint64
15 Type []byte
16 TypeEnum uint64 See table below
17 XferAsset uint64 Asset ID
18 AssetAmount uint64 value in Asset's units
19 AssetSender []byte 32 byte address. Causes clawback of all value of asset from AssetSender if Sender is the Clawback address of the asset.
20 AssetReceiver []byte 32 byte address
21 AssetCloseTo []byte 32 byte address
22 GroupIndex uint64 Position of this transaction within an atomic transaction group. A stand-alone transaction is implicitly element 0 in a group of 1
23 TxID []byte The computed ID for this transaction. 32 bytes.
24 ApplicationID uint64 ApplicationID from ApplicationCall transaction. LogicSigVersion >= 2.
25 OnCompletion uint64 ApplicationCall transaction on completion action. LogicSigVersion >= 2.
26 ApplicationArgs []byte Arguments passed to the application in the ApplicationCall transaction. LogicSigVersion >= 2.
27 NumAppArgs uint64 Number of ApplicationArgs. LogicSigVersion >= 2.
28 Accounts []byte Accounts listed in the ApplicationCall transaction. LogicSigVersion >= 2.
29 NumAccounts uint64 Number of Accounts. LogicSigVersion >= 2.
30 ApprovalProgram []byte Approval program. LogicSigVersion >= 2.
31 ClearStateProgram []byte Clear state program. LogicSigVersion >= 2.
32 RekeyTo []byte 32 byte Sender's new AuthAddr. LogicSigVersion >= 2.
33 ConfigAsset uint64 Asset ID in asset config transaction. LogicSigVersion >= 2.
34 ConfigAssetTotal uint64 Total number of units of this asset created. LogicSigVersion >= 2.
35 ConfigAssetDecimals uint64 Number of digits to display after the decimal place when displaying the asset. LogicSigVersion >= 2.
36 ConfigAssetDefaultFrozen uint64 Whether the asset's slots are frozen by default or not, 0 or 1. LogicSigVersion >= 2.
37 ConfigAssetUnitName []byte Unit name of the asset. LogicSigVersion >= 2.
38 ConfigAssetName []byte The asset name. LogicSigVersion >= 2.
39 ConfigAssetURL []byte URL. LogicSigVersion >= 2.
40 ConfigAssetMetadataHash []byte 32 byte commitment to some unspecified asset metadata. LogicSigVersion >= 2.
41 ConfigAssetManager []byte 32 byte address. LogicSigVersion >= 2.
42 ConfigAssetReserve []byte 32 byte address. LogicSigVersion >= 2.
43 ConfigAssetFreeze []byte 32 byte address. LogicSigVersion >= 2.
44 ConfigAssetClawback []byte 32 byte address. LogicSigVersion >= 2.
45 FreezeAsset uint64 Asset ID being frozen or un-frozen. LogicSigVersion >= 2.
46 FreezeAssetAccount []byte 32 byte address of the account whose asset slot is being frozen or un-frozen. LogicSigVersion >= 2.
47 FreezeAssetFrozen uint64 The new frozen value, 0 or 1. LogicSigVersion >= 2.

Additional details in the opcodes document on the txn op.

Global Fields

Global fields are fields that are common to all the transactions in the group. In particular it includes consensus parameters.

Index Name Type Notes
0 MinTxnFee uint64 micro Algos
1 MinBalance uint64 micro Algos
2 MaxTxnLife uint64 rounds
3 ZeroAddress []byte 32 byte address of all zero bytes
4 GroupSize uint64 Number of transactions in this atomic transaction group. At least 1
5 LogicSigVersion uint64 Maximum supported TEAL version. LogicSigVersion >= 2.
6 Round uint64 Current round number. LogicSigVersion >= 2.
7 LatestTimestamp uint64 Last confirmed block UNIX timestamp. Fails if negative. LogicSigVersion >= 2.
8 CurrentApplicationID uint64 ID of current application executing. Fails if no such application is executing. LogicSigVersion >= 2.

Asset Fields

Asset fields include AssetHolding and AssetParam fields that are used in asset_read_* opcodes

Index Name Type Notes
0 AssetBalance uint64 Amount of the asset unit held by this account
1 AssetFrozen uint64 Is the asset frozen or not
Index Name Type Notes
0 AssetTotal uint64 Total number of units of this asset
1 AssetDecimals uint64 See AssetParams.Decimals
2 AssetDefaultFrozen uint64 Frozen by default or not
3 AssetUnitName []byte Asset unit name
4 AssetName []byte Asset name
5 AssetURL []byte URL with additional info about the asset
6 AssetMetadataHash []byte Arbitrary commitment
7 AssetManager []byte Manager commitment
8 AssetReserve []byte Reserve address
9 AssetFreeze []byte Freeze address
10 AssetClawback []byte Clawback address

Flow Control

Op Description
err Error. Panic immediately. This is primarily a fencepost against accidental zero bytes getting compiled into programs.
bnz branch if value X is not zero
bz branch if value X is zero
b branch unconditionally to offset
return use last value on stack as success value; end
pop discard value X from stack
dup duplicate last value on stack
dup2 duplicate two last values on stack: A, B -> A, B, A, B

State Access

Op Description
balance get balance for the requested account specified by Txn.Accounts[A] in microalgos. A is specified as an account index in the Accounts field of the ApplicationCall transaction, zero index means the sender
app_opted_in check if account specified by Txn.Accounts[A] opted in for the application B => {0 or 1}
app_local_get read from account specified by Txn.Accounts[A] from local state of the current application key B => value
app_local_get_ex read from account specified by Txn.Accounts[A] from local state of the application B key C => {0 or 1 (top), value}
app_global_get read key A from global state of a current application => value
app_global_get_ex read from application Txn.ForeignApps[A] global state key B => {0 or 1 (top), value}. A is specified as an account index in the ForeignApps field of the ApplicationCall transaction, zero index means this app
app_local_put write to account specified by Txn.Accounts[A] to local state of a current application key B with value C
app_global_put write key A and value B to global state of the current application
app_local_del delete from account specified by Txn.Accounts[A] local state key B of the current application
app_global_del delete key A from a global state of the current application
asset_holding_get read from account specified by Txn.Accounts[A] and asset B holding field X (imm arg) => {0 or 1 (top), value}
asset_params_get read from asset Txn.ForeignAssets[A] params field X (imm arg) => {0 or 1 (top), value}

Assembler Syntax

The assembler parses line by line. Ops that just use the stack appear on a line by themselves. Ops that take arguments are the op and then whitespace and then any argument or arguments.

The first line may contain a special version pragma #pragma version X, which directs the assembler to generate TEAL bytecode targeting a certain version. For instance, #pragma version 2 produces bytecode targeting TEAL v2. By default, the assembler targets TEAL v1.

Subsequent lines may contain other pragma declarations (i.e., #pragma <some-specification>), pertaining to checks that the assembler should perform before agreeing to emit the program bytes, specific optimizations, etc. Those declarations are optional and cannot alter the semantics as described in this document.

"//" prefixes a line comment.

Constants and Pseudo-Ops

A few pseudo-ops simplify writing code. int and byte and addr followed by a constant record the constant to a intcblock or bytecblock at the beginning of code and insert an intc or bytec reference where the instruction appears to load that value. addr parses an Algorand account address base32 and converts it to a regular bytes constant.

byte constants are:

byte base64 AAAA...
byte b64 AAAA...
byte base64(AAAA...)
byte b64(AAAA...)
byte base32 AAAA...
byte b32 AAAA...
byte base32(AAAA...)
byte b32(AAAA...)
byte 0x0123456789abcdef...
byte "\x01\x02"
byte "string literal"

int constants may be 0x prefixed for hex, 0 prefixed for octal, or decimal numbers.

intcblock may be explictly assembled. It will conflict with the assembler gathering int pseudo-ops into a intcblock program prefix, but may be used if code only has explicit intc references. intcblock should be followed by space separated int constants all on one line.

bytecblock may be explicitly assembled. It will conflict with the assembler if there are any byte pseudo-ops but may be used if only explicit bytec references are used. bytecblock should be followed with byte constants all on one line, either 'encoding value' pairs (b64 AAA...) or 0x prefix or function-style values (base64(...)) or string literal values.

Labels and Branches

A label is defined by any string not some other op or keyword and ending in ':'. A label can be an argument (without the trailing ':') to a branch instruction.

Example:

int 1
bnz safe
err
safe:
pop

Encoding and Versioning

A program starts with a varuint declaring the version of the compiled code. Any addition, removal, or change of opcode behavior increments the version. For the most part opcode behavior should not change, addition will be infrequent (not likely more often than every three months and less often as the language matures), and removal should be very rare.

For version 1, subsequent bytes after the varuint are program opcode bytes. Future versions could put other metadata following the version identifier.

It is important to prevent newly-introduced transaction fields from breaking assumptions made by older versions of TEAL. If one of the transactions in a group will execute a TEAL program whose version predates a given field, that field must not be set anywhere in the transaction group, or the group will be rejected. For example, executing a TEAL version 1 program on a transaction with RekeyTo set to a nonzero address will cause the program to fail, regardless of the other contents of the program itself.

Varuint

A 'proto-buf style variable length unsigned int' is encoded with 7 data bits per byte and the high bit is 1 if there is a following byte and 0 for the last byte. The lowest order 7 bits are in the first byte, followed by successively higher groups of 7 bits.

What TEAL Cannot Do

Current design and implementation limitations to be aware of.

  • TEAL cannot create or change a transaction, only approve or reject.
  • Stateless TEAL cannot lookup balances of Algos or other assets. (Standard transaction accounting will apply after TEAL has run and authorized a transaction. A TEAL-approved transaction could still be invalid by other accounting rules just as a standard signed transaction could be invalid. e.g. I can't give away money I don't have.)
  • TEAL cannot access information in previous blocks. TEAL cannot access most information in other transactions in the current block. (TEAL can access fields of the transaction it is attached to and the transactions in an atomic transaction group.)
  • TEAL cannot know exactly what round the current transaction will commit in (but it is somewhere in FirstValid through LastValid).
  • TEAL cannot know exactly what time its transaction is committed.
  • TEAL cannot loop. Its branch instructions bnz "branch if not zero", bz "branch if zero" and b "branch" can only branch forward so as to skip some code.
  • TEAL cannot recurse. There is no subroutine jump operation.