「小技巧」取得網站的Root-CA

透過OpenSSl 可以方便取得指定網站的Root-CA 

openssl s_client -showcerts -connect japaneast.api.cognitive.microsoft.com:443

回應值如下

CONNECTED(00000005)
depth=2 C = IE, O = Baltimore, OU = CyberTrust, CN = Baltimore CyberTrust Root
verify return:1
depth=1 C = US, ST = Washington, L = Redmond, O = Microsoft Corporation, OU = Microsoft IT, CN = Microsoft IT TLS CA 2
verify return:1
depth=0 CN = *.cognitiveservices.azure.com
verify return:1
---
Certificate chain
 0 s:/CN=*.cognitiveservices.azure.com
   i:/C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/OU=Microsoft IT/CN=Microsoft IT TLS CA 2
-----BEGIN CERTIFICATE-----
MIIIxzCCBq+gAwIBAgITIAAFRAs77OdnDzBu6AAAAAVECzANBgkqhkiG9w0BAQsF
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7SzH4EHYXDrylkaWu9rH6LeThlmIhhzyrbeL/y4GwFdMPjc80zKNSXK0hw==
-----END CERTIFICATE-----
 1 s:/C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/OU=Microsoft IT/CN=Microsoft IT TLS CA 2
   i:/C=IE/O=Baltimore/OU=CyberTrust/CN=Baltimore CyberTrust Root
-----BEGIN CERTIFICATE-----
MIIFtDCCBJygAwIBAgIQDywQyVsGwJN/uNRJ+D6FaTANBgkqhkiG9w0BAQsFADBa
MQswCQYDVQQGEwJJRTESMBAGA1UEChMJQmFsdGltb3JlMRMwEQYDVQQLEwpDeWJl
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e0t+fCut38NMkTl8F0arflspaqUVVUov
-----END CERTIFICATE-----
---
Server certificate
subject=/CN=*.cognitiveservices.azure.com
issuer=/C=US/ST=Washington/L=Redmond/O=Microsoft Corporation/OU=Microsoft IT/CN=Microsoft IT TLS CA 2
---
No client certificate CA names sent
Server Temp Key: ECDH, P-384, 384 bits
---
SSL handshake has read 4513 bytes and written 358 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-AES256-GCM-SHA384
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : ECDHE-RSA-AES256-GCM-SHA384
    Session-ID: 996FFFB3854CAC1205B783A59B76BDFAE38D24425C77C1C50A8B80832D606261
    Session-ID-ctx: 
    Master-Key: BCD00D7E1211EBD9A8755BC67584AEFEAE435E1F8842B9D13F90F6C2F8E4CE0AA646FE75377DACA2DDC40323C70B2910
    TLS session ticket lifetime hint: 36000 (seconds)
    TLS session ticket:
    0000 - 00 00 00 00 f1 09 a9 bd-e1 cc 12 43 b9 30 10 64   ...........C.0.d
    0010 - 28 4e 9f b9 d4 14 fb 68-92 1b ea 73 13 94 94 d5   (N.....h...s....
    0020 - d5 27 00 73 cc c6 2d d8-76 e3 36 bf 65 5b c9 e4   .'.s..-.v.6.e[..
    0030 - af 7f 09 39 18 fc 7d d4-1c 4c c0 8c 11 e5 a4 c8   ...9..}..L......
    0040 - d4 24 8c 68 4d 74 12 12-1f 86 c6 a3 1c d2 27 14   .$.hMt........'.
    0050 - 4d 1d 3b 03 ec e9 ac d2-f8 d3 d1 53 76 60 5a 31   M.;........Sv`Z1
    0060 - 91 76 92 67 ce ee 5e 68-73 9c 4c a7 9a 55 93 a2   .v.g..^hs.L..U..
    0070 - 17 00 05 25 1a 25 e9 cb-32 97 31 23 fc 15 5d a5   ...%.%..2.1#..].
    0080 - 84 ae 5b 90 2e 9c e9 0f-63 5f 76 2e 61 4b 90 d9   ..[.....c_v.aK..
    0090 - a6 ab ca ea df 36 23 cc-41 85 54 13 4f cc 7b d6   .....6#.A.T.O.{.
    00a0 - a5 73 88 c5 2a 9c e7 ad-01 b3 0b 47 76 02 df 3a   .s..*......Gv..:
    00b0 - b4 49 f0 0f a0 4f ba ac-a5 c5 be 35 29 53 66 44   .I...O.....5)SfD
    00c0 - ec 8d ad 1a 1c d4 e6 55-e6 ae eb 9e 3e 9f 9f d5   .......U....>...
    00d0 - f0 11 0d 68 de 66 94 7e-18 82 c9 03 a8 ee b7 b2   ...h.f.~........
    00e0 - c0 08 aa a8 b4 48 61 cb-1f a5 1f b1 63 93 de f1   .....Ha.....c...
    00f0 - 8a f8 f4 23 6b 02 76 7f-7b ad b1 94 f7 93 80 f9   ...#k.v.{.......
    0100 - ea 26 28 82                                       .&(.

    Start Time: 1561476387
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
---

注意第二段 標示 CyberTrust Root 或是Root CA  即為所求

-----BEGIN CERTIFICATE-----
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e0t+fCut38NMkTl8F0arflspaqUVVUov
-----END CERTIFICATE-----

參考文章

http://www.iotsharing.com/2017/08/how-to-use-https-in-arduino-esp32.html