as-jwt

AssemblyScript package that provides simple jws handling for jwt tokens.

Installation:

npm install @gcoredev/as-jwt

Usage:

import { jwtVerify, JwtValidation } from "@gcoredev/as-jwt/assembly";

const token = "<base64-header>.<base64-payload>.<signed-secret>";
const secret = "my-long-private-key-that-is-hard-to-guess";

const result: JwtValidation = jwtVerify(token, secret);

if (result === JwtValidation.Ok) {
  // Jwt is valid.
} else {
  // Jwt validation failed
}

API

jwtVerify()

jwtVerify(token, secret, leewaySeconds?, validateIat?): JwtValidation

Validates token signature has been signed with a valid secret.

Also validates the time-based claims of RFC 7519 section 4.1:

Claim	Checked	Result when it fails
exp	always	Expired if the token has expired, BadToken if absent or not a NumericDate
nbf	when present	NotBefore if the token is not yet valid, BadToken if not a NumericDate
iat	only if validateIat	NotBefore if the token was issued in the future, BadToken if not a NumericDate

Clock skew

leewaySeconds (default 0) widens every time comparison in both directions, so signer and verifier clocks need not agree exactly. With the default of 0 a signer whose clock runs even slightly ahead of the verifier produces tokens that are rejected as NotBefore. Allow for the drift you actually expect:

// tolerate up to a minute of clock drift between signer and verifier
const result = jwtVerify(token, secret, 60);

Validating iat

validateIat (default false) is opt-in because RFC 7519 mandates no check on iat and treats it as informational. Left off, iat is ignored entirely. Turn it on to reject tokens issued in the future — but pair it with a leewaySeconds that covers your clock drift, since this check is the one most easily tripped by a fast signer clock:

const result = jwtVerify(token, secret, 60, true);

Accepted claim values

Time claims are compared in whole seconds and must be integers in the range 0 to 253402300799 (9999-12-31T23:59:59Z). A claim outside that range, or one that is not an integer, is rejected as BadToken. The bound is what keeps the comparisons from overflowing: an unbounded claim near the limit of a 64-bit integer wraps negative and slips past the check it should have failed.

leewaySeconds is clamped to that same range, and a negative value is normalised to 0. Note that this is a normalisation, not a guarantee of strictness: a negative tolerance has no meaning and is not honoured as a tightening, so passing one is equivalent to passing 0 and never verifies more strictly than 0 would.

Deviations from RFC 7519

• Only integer NumericDate values are accepted. RFC 7519 section 2 permits non-integer NumericDates, so a token carrying "exp": 1704070861.5 is spec-compliant but is rejected here as BadToken. It fails closed, and whole-second timestamps are what issuers emit in practice. This applies to exp and nbf always, but to iat only when validateIat is enabled — with it off, iat is not examined at all, so a malformed or out-of-range iat is simply ignored.
• exp is required, where RFC 7519 makes every claim optional.
• At the default leewaySeconds of 0, an exp equal to the current second is treated as expired, per RFC 7519 section 4.1.4, which requires the current time to be strictly before exp. A positive leewaySeconds deliberately relaxes that boundary rather than preserving it: the token is rejected only once now - exp >= leewaySeconds, giving an accepted grace window of [exp, exp + leewaySeconds). With leewaySeconds of 60, an exp up to 59 seconds in the past still verifies. The strict RFC boundary therefore applies only at 0.

For reference, the matching nbf boundary is inclusive at both ends: a token is rejected only once nbf - now > leewaySeconds, so an nbf exactly equal to the current second, or exactly leewaySeconds ahead of it, is accepted.

The iss, aud, sub and jti claims are not validated. If your application relies on them, decode the payload and check them yourself.

compactVerify()

compactVerify(token, secret): JwtValidation

Validates token signature has been signed with a valid secret.

Does NOT validate any claims in the payload.

Internal Libraries:

Under the hood this package is powered by:

• as-hmac-sha2 (ISC) — git submodule, unmodified, pinned to 1.0.3. Do not downgrade below 1.0.3: earlier revisions read multi-block messages from the wrong buffer offset and produce non-standard digests for signing inputs of 128 bytes (SHA-256) or 256 bytes (SHA-512) and above. The npm registry still serves 1.0.2, which is affected, so this dependency is pinned by commit rather than taken from npm.
• as-base64 (MIT) — git submodule, unmodified

See THIRD_PARTY_NOTICES.md for full license texts and a description of the modifications.