AwsEncryptionSdk.Stream.SignatureAccumulator (AWS Encryption SDK v0.7.0)

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Incremental signature accumulation for streaming ECDSA operations.

Purpose

Enables ECDSA signing/verification for large messages without buffering the entire message in memory. Used internally by streaming encryption and decryption for signed algorithm suites.

Memory Efficiency

Instead of buffering the entire message for signing:

  • Accumulates SHA-384 hash state incrementally
  • Hash state size is constant (64 bytes) regardless of message size
  • Final signature is computed from hash digest

This allows signing/verifying messages of any size with constant memory usage.

Signed Algorithm Suites

The AWS Encryption SDK includes algorithm suites with ECDSA P-384 signatures:

  • AES_256_GCM_HKDF_SHA512_COMMIT_KEY_ECDSA_P384 (0x0578, default)
  • AES_256_GCM_IV12_TAG16_HKDF_SHA384_ECDSA_P384 (0x0378)

For these suites, the entire message (header + all frames) is signed.

Usage Context

You typically don't use this module directly. It's used internally by:

Low-Level Example

If implementing custom streaming or signature logic:

# During encryption
acc = SignatureAccumulator.init()
acc = SignatureAccumulator.update(acc, header_bytes)
acc = SignatureAccumulator.update(acc, frame1_bytes)
acc = SignatureAccumulator.update(acc, frame2_bytes)
signature = SignatureAccumulator.sign(acc, private_key)

# During decryption
acc = SignatureAccumulator.init()
acc = SignatureAccumulator.update(acc, header_bytes)
acc = SignatureAccumulator.update(acc, frame1_bytes)
acc = SignatureAccumulator.update(acc, frame2_bytes)
valid? = SignatureAccumulator.verify(acc, signature, public_key)

Hash Algorithm

Uses SHA-384 for hash accumulation, matching the ECDSA P-384 curve used by signed algorithm suites.

See Also

Summary

Types

t()

Functions

digest(signature_accumulator)

Returns the current hash digest without finalizing.

init()

Initializes a new signature accumulator with SHA-384.

sign(signature_accumulator, private_key)

Finalizes the hash and signs with ECDSA P-384.

update(acc, data)

Updates the accumulator with additional data.

verify(signature_accumulator, signature, public_key)

Finalizes the hash and verifies an ECDSA P-384 signature.

Types

t()

@type t() :: %AwsEncryptionSdk.Stream.SignatureAccumulator{
  hash_ctx: :crypto.hash_state()
}

Functions

digest(signature_accumulator)

@spec digest(t()) :: binary()

Returns the current hash digest without finalizing.

Useful for debugging or intermediate verification.

init()

@spec init() :: t()

Initializes a new signature accumulator with SHA-384.

sign(signature_accumulator, private_key)

@spec sign(t(), binary()) :: binary()

Finalizes the hash and signs with ECDSA P-384.

Returns DER-encoded signature.

update(acc, data)

@spec update(t(), binary()) :: t()

Updates the accumulator with additional data.

verify(signature_accumulator, signature, public_key)

@spec verify(t(), binary(), binary()) :: boolean()

Finalizes the hash and verifies an ECDSA P-384 signature.

Returns true if valid, false otherwise.