purecrypt

Security notice. Pure Python cannot provide constant-time execution. Secret-key operations (signing, decryption, key exchange) are hardened against the obvious leaks: RSA blinding, fixed-iteration scalar multiplication with mask-selected adds, uniform error paths, and constant-time-style comparisons. Residual timing variance in the interpreter remains, so do not run secret-key operations where a co-located attacker can measure timing. Public-data operations such as certificate validation and signature verification are unaffected. For high-assurance deployments use audited native implementations such as cryptography (OpenSSL) or PyNaCl (libsodium).

Pure-Python cryptographic primitives for Python 3.10+. No dependencies, no Rust, no C extensions: only hashlib, hmac and secrets from the standard library.

Install

pip install purecrypt

Contents

• AES-128/192/256 with ECB, CBC, CTR and GCM (AEAD)
• ChaCha20, Poly1305, ChaCha20-Poly1305 and XChaCha20-Poly1305
• Ed25519 signatures and X25519 key exchange
• ECDSA and ECDH over P-256, P-384, P-521 and secp256k1 (RFC 6979 deterministic signatures)
• RSA: key generation, PSS and PKCS#1 v1.5 signatures, OAEP and PKCS#1 v1.5 encryption, PKCS#1/PKCS#8/SPKI serialization, encrypted PKCS#8 (PBES2/AES-CBC)
• X.509 certificate parsing, signature verification, RFC 5280 chain validation and RFC 6125 hostname matching
• HKDF, PBKDF2, scrypt and Argon2id
• Minimal DER/PEM handling for key serialization
• SHA-2, SHA-3, SHAKE, BLAKE2 and HMAC facades over hashlib

Example

from purecrypt.aes import gcm_decrypt, gcm_encrypt
from purecrypt.eddsa import Ed25519PrivateKey
from purecrypt.x509 import Certificate, validate_chain, verify_hostname

key, nonce = b"k" * 32, b"n" * 12
ct, tag = gcm_encrypt(key, nonce, b"secret", aad=b"hdr")
assert gcm_decrypt(key, nonce, ct, tag, aad=b"hdr") == b"secret"

priv = Ed25519PrivateKey.generate()
sig = priv.sign(b"message")
priv.public_key().verify(sig, b"message")

leaf = Certificate.from_pem(pem_text)
validate_chain(leaf, intermediates, trust_roots)
verify_hostname(leaf, "example.com")

Known limitations

• No constant-time guarantees. Python semantics preclude them.
• Secret zeroization is best-effort (mutable buffers are wiped where practical, but interpreter copies are out of scope).
• RSA PKCS#1 v1.5 decryption is inherently Bleichenbacher-prone. Use OAEP.
• X.509 validation covers RFC 5280 path rules and RFC 6125 hostnames, but there is no revocation checking (no CRL or OCSP) and no name or policy constraint enforcement. A critical extension whose semantics are not enforced fails validation.
• No TLS. That is a different and much larger problem.

Development

uv sync --group dev
make check

License: 0BSD. Quad4 Software, https://quad4.io