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• I read the RSA private key from ~/.ssh/id_rsa (default)
• Use the private key to encrypt a random 32 byte symmetric key (PKCS#1 OAEP)
• Encrypt a stream of data with AES-256 in counter mode (no extra padding)
• Write all of this to the output stream in 65535 byte chunks
• Append an HMAC-SHA256 of the ciphertext

• I read the RSA private key from ~/.ssh/id_rsa (default)
• Use the private key to encrypt a random 32 byte symmetric key (PKCS#1 OAEP)
• Encrypt a stream of data with AES-256 in counter mode (no extra padding)
• Write all of this to the output stream in 65535 byte chunks
• Append an HMAC-SHA256 of the ciphertext

Update: The project is now on GitHub

Changes so far

• I'm including the entire file header (magic bytes, encrypted key length, encrypted key) in the HMAC-SHA256 calculation
• I'm using a random 128 bit number as the initial value for the counter instead of effectively 64 bits with zeros appended.