JSEncrypt(js) encrypt, but python cannot decrypt

Question

I'm trying RSA encrypt text with JSEncrypt(javascript) and decrypt with python crypto (python3.7). Most of the time, it works. But sometimes, python cannot decrypt.

const encrypt = new JSEncrypt()
encrypt.setPublicKey(publicKey)
encrypt.encrypt(data)

from base64 import b64decode
from Crypto.Cipher import PKCS1_v1_5 as Cipher_PKCS1_v1_5
from Crypto.PublicKey import RSA

crypt_text = "J9I/IdsSGZqrQ5XBTlDrze5+U3otrGEGn7J7f330/tbIpdPNwu9k5gCh35HJHuRF6tXhbOD9XbHS6dGXwRdj0KNSWa43tDQMyGp/ZSewCd4wWkqIx83YzDKnYTVc9zWYbg2iYrmR03AqtWMysl8vZDUSmQn7gNdYEJGxSUzVng=="
private_key = "MIICXQIBAAKBgQClFImg7N+5ziGtjrMDwN7frootgwrLUmbE9YFBtecnjchCRjAn1wqq69XiWynEv0q3/U91N5g0nJxeMuolSM8cwdQbT3KZFwQF6vreSzDNhfEYOsFVZknILLPiJpUYm5w3Gi34UeM60iHGH9EUnmQeVwKSG0WF2nK2SCU6EyfoJwIDAQABAoGAHHk2Y/N3g2zykiUS64rQ5nQMkV0Q95D2+PH/oX3mqQPjjsrcc4K77E9RTQG8aps0IBgpJGa6chixP+44RMYSMvRIK0wqgX7s6AFIkFIIM+v+bP9pd3kKaVKTcNIjfnKJZokgAnU0QVdf0zeSNElZC+2qe1FbblsSQ6sqaFmHaMECQQC4oZO+w0q2smQh7VZbM0fSIbdZEimX/4y9KN4VYzPQZkDzQcEQX1Al2YAP8eqlzB4r7QcpRJgvUQDODhzMUtP9AkEA5ORFhPVK5slpqYP7pj2F+D2xAoL9XkgBKmhVppD/Sje/vg4yEKCTQ7fRlIzSvtwAvbDJi3ytYqXQWVdaD/Eb8wJAdYC3k8ecTCu6WHFA7Wf0hIJausA5YngMLPLObFQnTLFXErm9UlsmmgATZZJz4LLIXPJMBXKXXD20Qm9u2oa4TQJBAKxBopP6KiFfSNabDkLAoFb+znzuaZGPrNjmZjcRfh6zr+hvNHxQ7CMVbnNWO7AJT8FyD2ubK71GvnLOC2hd8sMCQQCT70B5EpFqULt7RBvCa7wwJsmwaMZLhBcfNmbry/J9SZG3FVrfYf15r0SBRug7mT2gRmH+tvt/mFafjG50VCnw"

decode_data = b64decode(crypt_text)
other_private_key = RSA.importKey(b64decode(private_key))
cipher = Cipher_PKCS1_v1_5.new(other_private_key)
decrypt_text = cipher.decrypt(decode_data, None).decode()
print(decrypt_text)

this is a example text that python can't decrypt, but js can decrypt it well. python throws the error:

File "/usr/local/lib/python3.7/site-packages/Crypto/Cipher/PKCS1_v1_5.py", line 165, in decrypt
    raise ValueError("Ciphertext with incorrect length.")
ValueError: Ciphertext with incorrect length.

Your `crypt_text` has two base64 encoded outputs concatenated together. — President James K. Polk, Jul 15 '19 at 13:08
Sorry, I have edited crypt_text. Could you give some suggestions?@JamesKPolk — eric.g, Jul 16 '19 at 02:23

score 3 · Accepted Answer · edited Oct 07 '21 at 11:04

If the ciphertext is Base64-decoded, the reason becomes clearer: The ciphertext doesn't have the length of the modulus (128 byte), but only 127 byte, i.e. it isn't padded to the length of the modulus with leading 0x00 values. This ciphertext is invalid (see RFC8017, step 1) and the decryption in the Python code fails with the error message Ciphertext with incorrect length. In contrast, the decryption in the JavaScript code works, i.e. JSEncrypt#decrypt obviously adjusts the ciphertext to the length of the modulus by stealthily padding with 0x00 values. If the ciphertext was created with JSEncrypt#encrypt, this method doesn't seem to work properly.

In detail: The modulus can be determined with:

openssl rsa -modulus -noout -in <path to private key>

and is (as hex-string):

A51489A0ECDFB9CE21AD8EB303C0DEDFAE8A2D830ACB5266C4F58141B5E7278DC842463027D70AAAEBD5E25B29C4BF4AB7FD4F753798349C9C5E32EA2548CF1CC1D41B4F7299170405EAFADE4B30CD85F1183AC1556649C82CB3E22695189B9C371A2DF851E33AD221C61FD1149E641E5702921B4585DA72B648253A1327E827

The length is 128 byte. The Base64-decoded ciphertext is (as hex-string):

27d23f21db12199aab4395c14e50ebcdee7e537a2dac61069fb27b7f7df4fed6c8a5d3cdc2ef64e600a1df91c91ee445ead5e16ce0fd5db1d2e9d197c11763d0a35259ae37b4340cc86a7f6527b009de305a4a88c7cdd8cc32a761355cf735986e0da262b991d3702ab56332b25f2f6435129909fb80d7581091b1494cd59e

The length is 127 byte. If the ciphertext is padded manually to the length of the modulus with 0x00-values, it can also be decrypted in the Python code:

0027d23f21db12199aab4395c14e50ebcdee7e537a2dac61069fb27b7f7df4fed6c8a5d3cdc2ef64e600a1df91c91ee445ead5e16ce0fd5db1d2e9d197c11763d0a35259ae37b4340cc86a7f6527b009de305a4a88c7cdd8cc32a761355cf735986e0da262b991d3702ab56332b25f2f6435129909fb80d7581091b1494cd59e

The decrypted data are:

Mzg4MDE1NDU4MTI1ODI0OA==NDQyODYwNjI1MjU4NTM2MA==

which are two valid Base64-encoded strings.

Your answer is fantastic！It works with padding '00' to the ciphertext. Thanks a lot! — eric.g, Jul 16 '19 at 09:19
That sounds like a bug in JSencrypt. Maybe file a bug report? — President James K. Polk, Jul 16 '19 at 12:26
@JamesKPolk - I've filed an issue [here](https://github.com/travist/jsencrypt/issues/158). — Topaco, Jul 16 '19 at 17:30

score 1 · Answer 2 · answered Jul 16 '19 at 09:22

Thanks to Topaco, it solved.

from base64 import b64decode, b16decode
from Crypto.Cipher import PKCS1_v1_5 as Cipher_PKCS1_v1_5
from Crypto.PublicKey import RSA

crypt_text = \
    "R247QGAFEeSW1wwXQuNf/cm/K/tnW5xwXLb5MuHW6/Fr8SRklM0n6Rmj07TgFwApeN72j/avXAvpoR70U92ehOJsDnnZguYN4u2bMXHDyTNmAXuJw9xPm59bSGcvgRm1X+V0Zq1FLzGEsPG6tOYEIX+wnIuH3P7QMd02XJfj0w0="
private_key = "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"

decode_data = b64decode(crypt_text)
if len(decode_data) == 127:
    hex_fixed = '00' + decode_data.hex()
    decode_data = b16decode(hex_fixed.upper())
other_private_key = RSA.importKey(b64decode(private_key))
cipher = Cipher_PKCS1_v1_5.new(other_private_key)
decrypt_text = cipher.decrypt(decode_data, None).decode()
print(decrypt_text)

JSEncrypt(js) encrypt, but python cannot decrypt

2 Answers2

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