当你有一个app，但是没有签名文件时，如何才能知道app的签名文件信息？
把apk后缀修改为zip，然后把META-INF目录复制出来，然后去JDK安装文件夹里，找到bin目录，在bin目录中cmd，使用命令执行：
C:\Program Files\Java\jdk1.7.0_60\bin>keytool -printcert -file E:\lll2018\M
ETA-INF\CERT.RSA
Owner: CN=Test CC CMBC, OU=kdf, O=sdfj, L=北京, ST=北京, C=86
Issuer: CN=Test CC CMBC, OU=kh, O=khjd, L=北京, ST=北京, C=86
Serial number: 40d612b
Valid from: Tue Jan 30 09:27:05 CST 2018 until: Sat Jan 24 09:27:05 CST 2043
Certificate fingerprints:
MD5: 44:1C:FB:3D:F2:30:52:12:E4:88:26:04:8D:19:D6:72
SHA1: 38:75:EE:0F:D9:12:13:5D:67:12:2C:8C:3D:07:4B:2C:C8:C2:DD:75
SHA256: D9:8C:CC:39:AD:C5:F1:85:3B:23:EA:33:42:1B:B0:D6:19:ED:10:EE:F9:
BF:25:B6:38:79:8E:38:34:49:46:96
Signature algorithm name: SHAwithRSA
Version: 3

Extensions:

1: ObjectId: 2.5.29.14 Criticality=false

SubjectKeyIdentifier [
KeyIdentifier [
0000: 65 F0 0E 15 04 0E 75 BD CB C1 FA F8 24 89 BE 04 e.....u.....$...
0010: C5 84 66 A7 ..f.
]
]

C:\Program Files\Java\jdk1.7.0_60\bin>