ios - Meaning of values received from "SecTrustCopyPublicKey" call -

when making https request google.com, did following:

import foundation  class learnnsurlsession: nsobject, nsurlsessiondelegate, nsurlsessiontaskdelegate {     override init() {         super.init()         let mysession = nsurlsession(configuration: nsurlsessionconfiguration.ephemeralsessionconfiguration(), delegate: self, delegatequeue: nsoperationqueue.mainqueue())         let data = mysession.datataskwithurl(nsurl(string: "https://www.google.com")!, completionhandler: myhandler)         data.resume()     }      func myhandler(data: nsdata!, response: nsurlresponse!, error: nserror!) -> void {         let s = nsstring(data: data, encoding: nsasciistringencoding)         println(s)     }      func urlsession(session: nsurlsession, task: nsurlsessiontask, didcompletewitherror error: nserror?) {         println("error:  \(error)")     }      // handles https connections     func urlsession(session: nsurlsession, didreceivechallenge challenge: nsurlauthenticationchallenge, completionhandler: (nsurlsessionauthchallengedisposition, nsurlcredential!) -> void) {         println("\npublic key:  \(sectrustcopypublickey(challenge.protectionspace.servertrust).takeunretainedvalue())")         completionhandler(nsurlsessionauthchallengedisposition.performdefaulthandling, nil)     }      // handles redirection     func urlsession(session: nsurlsession, task: nsurlsessiontask, willperformhttpredirection response: nshttpurlresponse, newrequest request: nsurlrequest, completionhandler: (nsurlrequest!) -> void) {         completionhandler(request)     } 

when run code, notice urlsession:didreceivechallenge gets called twice , following outputs println("public key:\(sectrustcopypublickey(challenge.protectionspace.servertrust).takeunretainedvalue())"):

public key:  <seckeyref algorithm id: 1, key type: rsapublickey, version: 3, block size: 2048 bits, exponent: {hex: 10001, decimal: 65537}, modulus: 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, addr: 0x7f96fb044000>  public key:  <seckeyref curve type: ksececcurvesecp256r1, algorithm id: 3, key type: ecpublickey, version: 3, block size: 256 bits, y: value1, x: value2, addr: 0x7f96fc032970> 

i have ommited values of "y" , "x" in second output, because don't know if should post them. "value1" (y) string 66 characters , "value2" (x) string 130 characters. both have numbers , upper-case letter, no symbols.

what "x" , "y" values in second output? "subjectpublickeyinfo"? why function called twice? , why must call "sectrustevaluate" before calling "sectrustcopypublickey"? documentation said must so, couldn't find why. also, ios automatically "sectrustevaluate" does, when https connection being established?

what "x" , "y" values in second output?

the parameters of elliptic curve being used. basically, "the public key." purposes, don't need know math, accept there 2 large numbers when put formula mathematical curve can used encrypt data sender can decrypt, or symmetrically, verify data sent given sender. if care, here's reasonably useful introduction concept. it's difficult explain without math, math in paper isn't crazy.

the "numbers , letters" hexidecimal encoding of 2 massive numbers. it's same "modulus" in other key list. that's different algorithm, called rsa. in case, that's (really incredibly huge) number used modulus (number divide , take remainder) in 1 of steps.

all of these numbers public. there no secrets here.

why function called twice?

it starts out negotiating rsa. looks upgrades elliptic curve, stronger given number of bits in key. short answer: "protocol upgrade."

and why must call "sectrustevaluate" before calling "sectrustcopypublickey"?

sectrustevaluate bunch of decoding necessary before public key available in data structure. "because security.framework implemented way." in theory, sectrustcopypublickey decoding if necessary, doesn't.

also, ios automatically "sectrustevaluate" does, when https connection being established?

yes.