ssh-keygen生成的RSA公钥在Java中转换报错问题排查

问题背景 #

使用ssh-keygen -b 2048 -t rsa生成RSA密钥对后，在Java Servlet中读取key_rsa.pub公钥文件时先后遇到两个错误：

1. 初始代码直接对完整公钥字符串做Base64解码，抛出Illegal base64 character错误
2. 移除公钥开头的ssh-rsa标识和末尾注释后，又在keyFactory.generatePublic(keySpec)处抛出java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: invalid key format

相关代码 #

Servlet读取公钥代码

@WebServlet(name = "Login", urlPatterns = "/login")
public class Login extends HttpServlet {
...
        InputStream publicKeyInputStream = getClass().getClassLoader().getResourceAsStream("key_rsa.pub");
        if (publicKeyInputStream == null) {
            out.println("fail - no public key");
            out.flush();
            return;
        }
        String publicKeyString = new String(publicKeyInputStream.readAllBytes(), StandardCharsets.UTF_8);
        try {
            PublicKey publicKey = util.RSAKeyGenerator.getPublicKey(publicKeyString);
...
        } catch (Exception e) {
...
        }
}

修改后的RSAKeyGenerator.getPublicKey方法

public static PublicKey getPublicKey(String publicKeyString) throws Exception {
    publicKeyString = publicKeyString.replaceAll("ssh-rsa\\s+", "").replaceAll("\\s.*","").trim();
    byte[] publicKeyBytes = Base64.getDecoder().decode(publicKeyString);
    X509EncodedKeySpec keySpec = new X509EncodedKeySpec(publicKeyBytes);
    KeyFactory keyFactory = KeyFactory.getInstance(RSA_ALGORITHM);
    return keyFactory.generatePublic(keySpec);
}

公钥文件内容

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCko/IRJIBoIN6fe3k8mCzsereXnQlTSwHutyMqZh7/JqbWJA31036I0YZUuOJ0d4WAMDWktWrfn7dkAlt+dqQtAlyzPUrqOc/euQ0iZPYhrd8Gn9jn+8WAQyt9L3LemgQ1ZWnXgNUk5Z1Y1vcbp8GsRWWLrxR7uvtbonU7QDJv5vEFnleObPqKeqobbg1iUoaXYkw0IAqhCTNW3ZvfWQic4FeHeNNVtT8XUrGyNPZkSgQ8QHsvSvNiLJ2bROhBD4AUZKBN/XRvNs+OSJM7agGroznHGxpwj7jZ0V+MH1HvHt95+4SnFAQJVfjza3lUPoMIOphq24V6hLzCr17lPECj kleber@kleber-desktop

问题原因 #

ssh-keygen生成的公钥是SSH格式的RSA公钥，而Java的X509EncodedKeySpec仅支持解析X.509标准格式的公钥编码，两者结构存在本质差异：

• SSH公钥的Base64部分封装了自定义结构：先包含算法标识（ssh-rsa）的字节长度与内容，再存储公钥的模和指数数据
• 直接解码SSH公钥的Base64内容，无法匹配X.509规范的密钥结构，因此抛出格式错误

解决方案 #

方案1：代码层面解析SSH公钥结构 #

修改getPublicKey方法，手动解析SSH公钥的内部结构，提取模和指数后构建标准RSA公钥：

public static PublicKey getPublicKey(String publicKeyString) throws Exception {
    // 移除前缀和末尾注释，保留纯Base64内容
    publicKeyString = publicKeyString.replaceAll("ssh-rsa\\s+", "").replaceAll("\\s.*", "").trim();
    byte[] decoded = Base64.getDecoder().decode(publicKeyString);
    
    try (ByteArrayInputStream bais = new ByteArrayInputStream(decoded);
         DataInputStream dis = new DataInputStream(bais)) {
        // 跳过算法标识部分（长度+内容）
        int algLen = dis.readInt();
        byte[] algBytes = new byte[algLen];
        dis.readFully(algBytes);
        
        // 读取公钥模
        int modLen = dis.readInt();
        byte[] modBytes = new byte[modLen];
        dis.readFully(modBytes);
        BigInteger modulus = new BigInteger(1, modBytes);
        
        // 读取公钥指数
        int expLen = dis.readInt();
        byte[] expBytes = new byte[expLen];
        dis.readFully(expBytes);
        BigInteger exponent = new BigInteger(1, expBytes);
        
        // 构建标准RSA公钥规范并生成公钥
        RSAPublicKeySpec spec = new RSAPublicKeySpec(modulus, exponent);
        KeyFactory keyFactory = KeyFactory.getInstance(RSA_ALGORITHM);
        return keyFactory.generatePublic(spec);
    }
}

方案2：提前转换公钥格式 #

用工具将SSH公钥转换为X.509格式，之后直接用原代码读取：

1. 执行以下命令转换公钥：

ssh-keygen -f key_rsa.pub -e -m PKCS8 > key_rsa_x509.pub

2. 将转换后的key_rsa_x509.pub放入项目资源目录，原getPublicKey方法无需修改即可正常解析

内容的提问来源于stack exchange，提问作者Kleber Mota