RSA Decrypt

About this tool

RSA Decrypt recovers the plaintext from a single RSA ciphertext block with the matching private key. Use it when a message or wrapped symmetric key was encrypted with RSA-OAEP or legacy RSAES-PKCS1-v1_5 and you need to verify the payload locally. Paste a PEM private key, paste the base64 or hex ciphertext, choose the padding and OAEP hash that the sender used, and select whether the recovered bytes should be rendered as UTF-8 text, hex, or base64.

The private key and ciphertext are processed in WebAssembly; the tool does not upload them. OAEP with SHA-256 is the default because it is the modern RSA encryption mode. PKCS#1 v1.5 is included only for compatibility with old systems. RSA can decrypt only one key-sized block, so large messages should be handled with hybrid encryption: RSA decrypts a small random content key, then a symmetric cipher decrypts the bulk data.

Worked example

The bundled example ciphertext was encrypted with OAEP-SHA256 to the throwaway fixture key used by this repository's tests. Paste that fixture private key, select padding = oaep, hash = sha256, ciphertext_encoding = base64, and output_encoding = utf8; the plaintext is hello from rsa-decrypt. For binary payloads, switch output_encoding to hex or base64 instead of trying to force the bytes into text.

Limits and edge cases

• The ciphertext must be exactly one RSA block: 256 bytes for a 2048-bit key, 512 bytes for a 4096-bit key, and so on.
• OAEP decryption succeeds only when the hash setting matches the sender's OAEP/MGF1 hash.
• PKCS#8 and PKCS#1 PEM private keys are accepted. Encrypted PKCS#8 keys require a passphrase; legacy OpenSSL Proc-Type: 4,ENCRYPTED PEM keys must be converted to PKCS#8 first.
• If the decrypted bytes are not valid UTF-8, choose hex or base64 output.
• This tool does not generate keys, encrypt messages, or perform signatures. Use the neighboring RSA encrypt/sign/verify tools for those surfaces.

FAQ