RSA decryption according to the private key reports an error

Question

My code using RSA on the front end：

const rsa = new JSEncrypt();
rsa.setPublicKey(k);
const resultText = rsa.encrypt("violet");
console.log(resultText);

My code using RSA in the backend：

byte[] inputByte = org.apache.commons.codec.binary.Base64.decodeBase64(str.getBytes("UTF-8"));
byte[] decoded = org.apache.commons.codec.binary.Base64.decodeBase64(privateKey);
PrivateKey priKey = KeyFactory.getInstance("RSA").generatePrivate(new 
PKCS8EncodedKeySpec(decoded));
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.DECRYPT_MODE,priKey);
String outStr=new String(cipher.doFinal(inputByte));
return outStr;

PublicKey like this:

PrivateKey like this:

Bu-t, when i do the java code decryption, it reported such an error：

java.security.InvalidKeyException: IOException : DerInputStream.getLength(): lengthTag=111, too big.

How can i solve this problem ?

You have just compromised your private key. You must now generate a new keypair and stop using this one. ***NEVER*** post your private key ***anywhere***. — user207421, Apr 25 '22 at 03:27

score 0 · Accepted Answer · answered Apr 25 '22 at 06:46

1. You are decoding wrong. PEM format has a dash-BEGIN line identifying the type of data, a block of base64 encoding the data, and a dash-END line. The BEGIN and END lines are part of the format, but they do not contain base64-encoded data; only the lines in between contain the base64-encoded data. You are apparently passing the whole thing, including the BEGIN and END lines, to commons.codec.Base64, which results in decoding a bunch of garbage before and after the actual data. That garbage isn't valid ASN.1 DER, so when Java tries to parse it as DER it fails.

2. Plus your data is not a PKCS8-clear privatekey. The PEM type 'RSA PRIVATE KEY' is an OpenSSL-defined format that contains a 'traditional' or 'legacy' format, namely the PKCS1 representation of the private key. This is not PKCS8, which is the only key format Java supports natively; that's why the spec class is named PKCS8EncodedKeySpec, because it is a key spec encoded as PKCS8 and more specifically PKCS8-clear. If you fix the above problem by removing the BEGIN and END lines before base64-decoding, Java can parse the result as DER, but not as a PKCS8-clear key; you get a different exception about 'algid parse error, not a sequence'. To fix this there are 5 approaches:

change whatever process you use to initially generate the keypair so it generates PKCS8, not OpenSSL-legacy PKCS1. Especially since you need anyway to replace the keypair you compromised by publishing it, as 207421 said. You give no clue what that process is or was, so I can't give any details.
convert your generated privatekey, or a copy, to PKCS8-clear. This is not programming or development and offtopic, but if you have or get OpenSSL (on the same or any accessible and secure system), you can do
```
openssl pkey -in oldfile -out newfile   # 1.0.0 up only, but older is now rare
# or
openssl pkcs8 -topk8 -nocrypt -in oldfile -out newfile   # even ancient versions
```

Once you have a PKCS8-clear file, just remove the BEGIN and END lines and base64-decode what is left, and pass that to KeyFactory as PKCS8EncodedKeySpec as you already do.

use https://www.bouncycastle.org . The 'bcpkix' jar has (Java) code to read a large range of OpenSSL-supported PEM formats, including the RSA-PKCS1 private key format you have. There are lots of existing Qs about this; just search for PEMParser and JcaPEMKeyConverter.
convert it yourself. Decode the body of the file you have, after removing the BEGIN and END lines, to get the PKCS1 key, then build the PKCS8 format for that key, and then pass it to KeyFactory as PKCS8EncodedKeySpec. See answers by Noa Resare and Jean-Alexis Aufauvre on Getting RSA private key from PEM BASE64 Encoded private key file or mine in Java: Convert DKIM private key from RSA to DER for JavaMail .
do it entirely yourself. Decode the file you have without BEGIN/END to get PCKS1, parse that as DER following e.g. RFC8447, and build RSAPrivateCrtKeySpec. Some other As on the Q I linked just above do this. However, this requires either: using undocumented internal sun.* classes, which used to work in Java (hence the existing answers) but which 'modular' Java versions (9 up) since 2017 have steadily made more difficult or impossible; using BouncyCastle which has documented (and good) support for ASN.1 -- but then it's easier to use bcpkix for the whole job as above; or writing your own ASN.1 parsing, which is a good deal of work.

PS: encrypting text with RSA is usually a bad design; it's not suited for that. But that's not really a programming issue and doesn't belong here.

Thank you very much for your detailed answer based on this question. It may be because I am new to it, so I am groping for it, and the field of vision is not so wide. If possible, do you have a recommended encryption method ? I will practice it，thanks！ — Chandler, Apr 26 '22 at 02:57
This problem is slightly modified according to your explanation, it has been solved, thank you！ — Chandler, Apr 26 '22 at 02:58

RSA decryption according to the private key reports an error

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