Quelle: https://www.cryer.co.uk/glossary/r/rsa/mathematical_guts_of_rsa_encryption.html

The RSA algorithm was invented in 1978 by Ron Rivest, Adi Shamir, and Leonard Adleman.

Here's the relatively easy to understand math behind RSA public key encryption.

1. Find P and Q, two large (e.g., 1024-bit) prime numbers.
2. Choose E such that E is greater than 1, E is less than PQ, and E and (P-1)(Q-1) are relatively prime, which means they have no prime factors in common. E does not have to be prime, but it must be odd. (P-1)(Q-1) can't be prime because it's an even number.
3. Compute D such that (DE - 1) is evenly divisible by (P-1)(Q-1). Mathematicians write this as DE = 1 (mod (P-1)(Q-1)), and they call D the multiplicative inverse of E. This is easy to do – simply find an integer X which causes D = (X(P-1)(Q-1) + 1)/E to be an integer, then use that value of D.
4. The encryption function is C = (T^E) mod PQ, where C is the ciphertext (a positive integer), T is the plaintext (a positive integer), and ^ indicates exponentiation. The message being encrypted, T, must be less than the modulus, PQ.
5. The decryption function is T = (C^D) mod PQ, where C is the ciphertext (a positive integer), T is the plaintext (a positive integer), and ^ indicates exponentiation.

Your public key is the pair (PQ, E). Your private key is the number D (reveal it to no one). The product PQ is the modulus (often called N in the literature). E is the public exponent. D is the secret exponent.

You can publish your public key freely, because there are no known easy methods of calculating D, P, or Q given only (PQ, E) (your public key). P and Q need to be sufficiently long (>2048 bits), for this method to be considered secure.

Here is an example of RSA encryption:

P  = 61    <- first prime number (destroy this after computing E and D)
Q  = 53    <- second prime number (destroy this after computing E and D)
PQ = 3233  <- modulus (give this to others)
E  = 17    <- public exponent (give this to others)
D  = 2753  <- private exponent (keep this secret!)

Your public key is (E,PQ).
Your private key is D.

The encryption function is:

	encrypt(T) = (T^E) mod PQ
		   = (T^17) mod 3233

The decryption function is:

	decrypt(C) = (C^D) mod PQ
		   = (C^2753) mod 3233

To encrypt the plaintext value 123, do this:

	encrypt(123) = (123^17) mod 3233
		     = 337587917446653715596592958817679803 mod 3233
		     = 855

To decrypt the ciphertext value 855, do this:

	decrypt(855) = (855^2753) mod 3233
		     = 123

One way to compute the value of 855^2753 mod 3233 is like this:

2753 = 101011000001 base 2, therefore

	2753 = 1 + 2^6 + 2^7 + 2^9 + 2^11
	     = 1 + 64  + 128 + 512 + 2048

Consider this table of powers of 855:

	855^1 = 855 (mod 3233)
	855^2 = 367 (mod 3233)
	855^4 = 367^2 (mod 3233) = 2136 (mod 3233)
	855^8 = 2136^2 (mod 3233) = 733 (mod 3233)
	855^16 = 733^2 (mod 3233) = 611 (mod 3233)
	855^32 = 611^2 (mod 3233) = 1526 (mod 3233)
	855^64 = 1526^2 (mod 3233) = 916 (mod 3233)
	855^128 = 916^2 (mod 3233) = 1709 (mod 3233)
	855^256 = 1709^2 (mod 3233) = 1282 (mod 3233)
	855^512 = 1282^2 (mod 3233) = 1160 (mod 3233)
	855^1024 = 1160^2 (mod 3233) = 672 (mod 3233)
	855^2048 = 672^2 (mod 3233) = 2197 (mod 3233)

Given the above, we know this:

	855^2753 (mod 3233)
	= 855^(1 + 64  + 128 + 512 + 2048) (mod 3233)
	= 855^1 * 855^64 * 855^128 * 855^512 * 855^2048 (mod 3233)
	= 855 * 916 * 1709 * 1160 * 2197 (mod 3233)
	= 794 * 1709 * 1160 * 2197 (mod 3233)
	= 2319 * 1160 * 2197 (mod 3233)
	= 184 * 2197 (mod 3233)
	= 123 (mod 3233)
	= 123

If you have a computer program (such as the "bc" utility that comes with Linux),
you can compute 855^2753 mod 3233 directly, like this:

	855^2753 mod 3233 
	= 50432888958416068734422899127394466631453878360035509315554967564501
	  05562861208255997874424542811005438349865428933638493024645144150785
	  17209179665478263530709963803538732650089668607477182974582295034295
	  04079035818459409563779385865989368838083602840132509768620766977396
	  67533250542826093475735137988063256482639334453092594385562429233017
	  51977190016924916912809150596019178760171349725439279215696701789902
	  13430714646897127961027718137839458696772898693423652403116932170892
	  69617643726521315665833158712459759803042503144006837883246101784830
	  71758547454725206968892599589254436670143220546954317400228550092386
	  36942444855973333063051607385302863219302913503745471946757776713579
	  54965202919790505781532871558392070303159585937493663283548602090830
	  63550704455658896319318011934122017826923344101330116480696334024075
	  04695258866987658669006224024102088466507530263953870526631933584734
	  81094876156227126037327597360375237388364148088948438096157757045380
	  08107946980066734877795883758289985132793070353355127509043994817897
	  90548993381217329458535447413268056981087263348285463816885048824346
	  58897839333466254454006619645218766694795528023088412465948239275105
	  77049113329025684306505229256142730389832089007051511055250618994171
	  23177795157979429711795475296301837843862913977877661298207389072796
	  76720235011399271581964273076407418989190486860748124549315795374377
	  12441601438765069145868196402276027766869530903951314968319097324505
	  45234594477256587887692693353918692354818518542420923064996406822184
	  49011913571088542442852112077371223831105455431265307394075927890822
	  60604317113339575226603445164525976316184277459043201913452893299321
	  61307440532227470572894812143586831978415597276496357090901215131304
	  15756920979851832104115596935784883366531595132734467524394087576977
	  78908490126915322842080949630792972471304422194243906590308142893930
	  29158483087368745078977086921845296741146321155667865528338164806795
	  45594189100695091965899085456798072392370846302553545686919235546299
	  57157358790622745861957217211107882865756385970941907763205097832395
	  71346411902500470208485604082175094910771655311765297473803176765820
	  58767314028891032883431850884472116442719390374041315564986995913736
	  51621084511374022433518599576657753969362812542539006855262454561419
	  25880943740212888666974410972184534221817198089911953707545542033911
	  96453936646179296816534265223463993674233097018353390462367769367038
	  05342644821735823842192515904381485247388968642443703186654199615377
	  91396964900303958760654915244945043600135939277133952101251928572092
	  59788751160195962961569027116431894637342650023631004555718003693586
	  05526491000090724518378668956441716490727835628100970854524135469660
	  84481161338780654854515176167308605108065782936524108723263667228054
	  00387941086434822675009077826512101372819583165313969830908873174174
	  74535988684298559807185192215970046508106068445595364808922494405427
	  66329674592308898484868435865479850511542844016462352696931799377844
	  30217857019197098751629654665130278009966580052178208139317232379013
	  23249468260920081998103768484716787498919369499791482471634506093712
	  56541225019537951668976018550875993133677977939527822273233375295802
	  63122665358948205566515289466369032083287680432390611549350954590934
	  06676402258670848337605369986794102620470905715674470565311124286290
	  73548884929899835609996360921411284977458614696040287029670701478179
	  49024828290748416008368045866685507604619225209434980471574526881813
	  18508591501948527635965034581536416565493160130613304074344579651083
	  80304062240278898042825189094716292266898016684480963645198090510905
	  79651307570379245958074479752371266761011473878742144149154813591743
	  92799496956415653866883891715446305611805369728343470219206348999531
	  91764016110392490439179803398975491765395923608511807653184706473318
	  01578207412764787592739087492955716853665185912666373831235945891267
	  87095838000224515094244575648744840868775308453955217306366938917023
	  94037184780362774643171470855830491959895146776294392143100245613061
	  11429937000557751339717282549110056008940898419671319709118165542908
	  76109008324997831338240786961578492341986299168008677495934077593066
	  02207814943807854996798945399364063685722697422361858411425048372451
	  24465580270859179795591086523099756519838277952945756996574245578688
	  38354442368572236813990212613637440821314784832035636156113462870198
	  51423901842909741638620232051039712184983355286308685184282634615027
	  44187358639504042281512399505995983653792227285847422071677836679451
	  34363807086579774219853595393166279988789721695963455346336497949221
	  13017661316207477266113107012321403713882270221723233085472679533015
	  07998062253835458948024820043144726191596190526034069061930939290724
	  10284948700167172969517703467909979440975063764929635675558007116218
	  27727603182921790350290486090976266285396627024392536890256337101471
	  68327404504583060228676314215815990079164262770005461232291921929971
	  69907690169025946468104141214204472402661658275680524166861473393322
	  65959127006456304474160852916721870070451446497932266687321463467490
	  41185886760836840306190695786990096521390675205019744076776510438851
	  51941619318479919134924388152822038464729269446084915299958818598855
	  19514906630731177723813226751694588259363878610724302565980914901032
	  78384821401136556784934102431512482864529170314100400120163648299853
	  25166349056053794585089424403855252455477792240104614890752745163425
	  13992163738356814149047932037426337301987825405699619163520193896982
	  54478631309773749154478427634532593998741700138163198116645377208944
	  00285485000269685982644562183794116702151847721909339232185087775790
	  95933267631141312961939849592613898790166971088102766386231676940572
	  95932538078643444100512138025081797622723797210352196773268441946486
	  16402961059899027710532570457016332613431076417700043237152474626393
	  99011899727845362949303636914900881060531231630009010150839331880116
	  68215163893104666659513782749892374556051100401647771682271626727078
	  37012242465512648784549235041852167426383189733332434674449039780017
	  84689726405462148024124125833843501704885320601475687862318094090012
	  63241969092252022679880113408073012216264404133887392600523096072386
	  15855496515800103474611979213076722454380367188325370860671331132581
	  99227975522771848648475326124302804177943090938992370938053652046462
	  55147267884961527773274119265709116613580084145421487687310394441054
	  79639308530896880365608504772144592172500126500717068969428154627563
	  70458838904219177398190648731908014828739058159462227867277418610111
	  02763247972904122211994117388204526335701759090678628159281519982214
	  57652796853892517218720090070389138562840007332258507590485348046564
	  54349837073287625935891427854318266587294608072389652291599021738887
	  95773647738726574610400822551124182720096168188828493894678810468847
	  31265541726209789056784581096517975300873063154649030211213352818084
	  76122990409576427857316364124880930949770739567588422963171158464569
	  84202455109029882398517953684125891446352791897307683834073696131409
	  74522985638668272691043357517677128894527881368623965066254089894394
	  9516191200216077789887686473648183782532484669916837281220310791935
	 64666840159148582699993374427677252"75403853322196852298590851548110
	  4022965'916338257385513314823459591633281445819843614596306024993617
	  5309792556123803901469066516347371885958277252568311998984646025"16	
	  46279764077057034806406450769779869955106180046471937808223250148934
	  078511378332510737538234034662695532926088138438957840)9804170410417
	  57608463062862610614059615207066695243018438575031762939543026312673
	  77406936404705896083462601885911184367532529845888040849710922999195
	  65539701911191919188327308603766775339607722455632113506572191067587
	  51186812786344197572392195263333856538388240057190102564949233944519
	  65959203992392217400247234147190970964562108299547746193228981181286
	  05556588093851898811812905614274085809168765711911224763288658712755
	  38928438126611991937924624112632990739867854558756652453056197509891
	  14578114735771283607554001774268660965093305172102723066635739462334
	  13638045914237759965220309418558880039496755829711258361621890140359
	  54234930424749053693992776114261796407100127643280428706083531594582
	  305946326827861270203356980346143245697021484375 mod 3233
	= 123