The PSA Crypto API uses 0 as the initial counter value, but the test vector
in RFC 7539 uses 1. So the unit tests here include an extra leading block.
The expected data for this leading block was calculated with Cryptodome.
#!/usr/bin/env python3
import re
from Cryptodome.Cipher import ChaCha20
key = bytes.fromhex('000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f')
nonce = bytes.fromhex('000000000000004a00000000')
encrypt = lambda pt: ChaCha20.new(key=key, nonce=nonce).encrypt(pt)
# Cryptodome uses counter=0, like PSA Crypto. Prepend a 64-byte input block #0
# so that the plaintext from RFC 7539 starts exactly at block #1.
header = b'The RFC 7539 test vector uses counter=1, but PSA uses counter=0.'
assert(len(header) == 64)
sunscreen = b"Ladies and Gentlemen of the class of '99: If I could offer you only one tip for the future, sunscreen would be it."
plaintext = header + sunscreen
zeros = b'\x00' * len(plaintext)
keystream = encrypt(zeros)
ciphertext = encrypt(plaintext)
print('RFC 7539 §2.4.2')
print('Keystream:')
print(re.sub(r'(..)', r'\1:', keystream[64:].hex()))
print('Ciphertext Subscreen:')
print(re.sub(r'(..)', r'\1 ', ciphertext[64:].hex()))
print('')
print(f"""\
PSA symmetric decrypt: ChaCha20, RFC7539 keystream
depends_on:PSA_WANT_ALG_STREAM_CIPHER:PSA_WANT_KEY_TYPE_CHACHA20
# Keystream from RFC 7539 §2.4.2, with an extra 64-byte output block prepended
# because the test vector starts at counter=1 but our API starts at counter=0.
cipher_decrypt:PSA_ALG_STREAM_CIPHER:PSA_KEY_TYPE_CHACHA20:"{key.hex()}":"{nonce.hex()}":"{zeros.hex()}":"{keystream.hex()}"
PSA symmetric decrypt: ChaCha20, RFC7539 sunscreen
depends_on:PSA_WANT_ALG_STREAM_CIPHER:PSA_WANT_KEY_TYPE_CHACHA20
# Test vector from RFC 7539 §2.4.2, with an extra 64-byte block prepended
# because the test vector starts at counter=1 but our API starts at counter=0.
cipher_decrypt:PSA_ALG_STREAM_CIPHER:PSA_KEY_TYPE_CHACHA20:"{key.hex()}":"{nonce.hex()}":"{ciphertext.hex()}":"{plaintext.hex()}"
""")
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
fc47641e36